An engine is a machine that converts energy into force and motion. Possible sources of energy include heat, chemical energy in a fuel, nuclear energy, and solar radiation. The force and motion usually take the form of output torque delivered to a rotating shaft. (Torque is the twisting effort developed around a center of rotation. In an engine it is conceptually quantified by the product of the radius from the center of the output shaft to a second point at which a tangential force is applied, and the magnitude of that applied tangential force.) In contrast, the output of a jet or a rocket engine is simply the thrust force derived from its high-velocity exhaust jet.

Most often the input energy to an engine is derived through combustion of a fuel. The result is a combustion engine. Combustion engines can be classified according to the nature of their...